The present disclosure relates to a method for producing a press-hardened molded part having at least two regions that differ regarding their strength, including at least one region of lower strength and at least one region of higher strength. This is accomplished by means of a press-hardening tool in which a blank, which has been heated to a forming temperature, is formed and held in the tool for the purpose of heat treating. To adjust the strength, at least one region of lower strength is cooled slower as compared to the regions that are to have higher strength. The present disclosure furthermore relates to a press-hardening tool with a forming surface for producing a press-hardened molded part having at least two regions that differ regarding their strength.
Press-hardening tools are tools for forming and hardening metal blanks. For the purposes of hardening, depending on the configuration of the method, an appropriately preheated blank is fed into the press-hardening tool in which it is formed and then sufficiently quickly cooled and thus hardened after the forming has been completed. Such press-hardening tools for example are used for producing structural components of motor vehicles. The blanks are normally steel sheet blanks.
There are numerous applications in which a structural component for a motor vehicle with different mechanical properties is desired. Such structural component can be the B-pillar of a motor vehicle, for example. For a B-pillar, it is sometimes desired for the column to have less strength in the region of the base and thus increased ductility, compared to the connecting regions of higher strength and a correspondingly lower ductility.
Such a B-pillar is known from EP 1 180 470 B1, which for the purpose of adjusting the different strengths is differently heated in regions prior to introducing the blank. For this purpose, those regions of the blank in which the formed molding must have increased strength are heated to a higher temperature than those which must have lower mechanical strength properties. The method known from EP 1 180 470 B1 uses a steel blank in which the regions of higher strength in the molded part, i.e. the B-pillar, is heated to austenitizing temperature, while those regions of the blank corresponding to the regions of the formed molded part of lower strength are only heated to a temperature which is distinctly below the austenitizing temperature. In order to protect the regions that are to be heated to a lower temperature, EP 1 180 470 B1 proposes to insulate these regions. It has been shown, however, that this manner of handling the blanks is complicated. At the same time, it must be recognized that a certain amount of lack of definition of the dimensional accuracy in the delimitation of the regions of higher strength has to be accepted.
WO 2006/038868 A1 discloses an additional method for producing press-hardened moldings with regions of different strength. In this method, the tool design is configured such that those regions of the blank to be provided with a lower strength are located in a tool cavity. To hold the molded part in the tool in the regions of lower strength, support structures are incorporated into the cavity, which are supported on the molded part. The contact area between the molding surface of the tool and the molded part within such cavity amounts to less than 25% of the surface area. It is possible to produce structural components having regions of different strength using the method and the press-hardening tool described in this document, wherein the blank was previously heated uniformly to its forming temperature. However, the dimensional accuracy of the molded parts produced with this method and/or with this tool is frequently not sufficient. Especially with structural components in which seals are to be used, as is the case with a B-pillar, for example, the molded parts produced with this method do not meet the necessary tolerance requirements. In the methods known from the prior art, those regions of the molding are moreover limited with respect to their size which have a lower strength throughout, since regions of higher strengths are formed in the contact areas.